The present invention relates generally to a package suitable for encapsulating electronic devices which must be cooled by a cryogen and, more particularly, to a package suitable for encapsulating superconducting devices, such as Josephson devices.
Recently, a superconducting integrated circuit for a high-speed computer, incorporating superconducting devices, such as Josephson junction devices using Nb have been developed. The Josephson effect is very important in the application of superconductors to the field of electronics. A SQUID (superconducting quantum interference device) for high-sensitivity magnetic field detection, as well as the superconducting integrated circuit, employs the Josephson effect. Currents that flow across a junction of a superconductor/insulator/superconductor structure are classified into Josephson currents that flow without voltage drop, and currents that flow with a finite voltage Vg corresponding to the band gap of the superconductor. The Josephson junction device can be used as a device for logical operation by representing a binary digit "0" by 0 volt, and a binary digit "1" by the finite voltage Vg. Magnetic flux .phi. arrested by a superconducting loop including the junction is quantized by a magnetic flux quantum .phi..sub.0 as the unit and is expressed by .phi.=n .phi..sub.0 (n is an integer). The Josephson junction device can be used as a digital storage device by representing binary digits "0" and "1" respectively by a state where n=-1 and a state where n=1.
A logic circuit employing superconducting devices, such as Josephson junction devices, is capable of operating at a high speed at a low power consumption, and hence a high-speed processor can easily be realized by the employment of such a logic circuit. High-speed signal exchange between such a high-speed processor and electronic devices, such as LSIs, disposed outside a low-temperature atmosphere must be possible for the effective use of the high-speed processor, or it is impossible to make the most of the advantages of the high-speed processor even if only the high-speed processor is capable of operating at a high operating speed. The arrangement of a package encapsulating superconducting devices is very important for high-speed signal transmission.
Generally, liquid helium (He) is used as a cryogen to cool the superconducting device for operation. The superconducting device is immersed in liquid He contained in a Dewar vessel, and is connected electrically by a coaxial cable to other semiconductor devices placed in a room temperature.
In such an arrangement of the superconducting devices and the semiconductor devices, the lenght of the coaxial cable must be at least about 1 m, and hence the estimated propagation delay is on the order of 10 ns. It is impossible to drive the superconducting devices, which operate at a high speed, for example, 1 ns or less, when the propagation delay is as large as 10 ns.